Capacitance type seat occupant sensor system

Abstract

A capacitance type seat occupant sensor system of a vehicle includes an AC electric power source, a main electrode disposed at a seat of the vehicle to form an electric field between the main electrode and the body of the vehicle, a guard electrode disposed between a seat frame and the main electrode, a switch unit for establishing connection of the guard electrode to have the same potential as the main electrode to provide a seat occupant sensing mode, an impedance calculating unit for calculating an impedance of a sensing circuit based on the AC voltage and an amount of current flowing through the sensing circuit, a real-part-and-imaginary-part calculating unit and judging unit for discriminating an occupant on the seat based on at least the imaginary part of the first impedance. The impedance calculating unit calculates a first impedance of an electric circuit including the main electrode when the switch means provides the seat occupant sensing mode, and the real-part-and-imaginary-part calculating unit calculates the real part and the imaginary part of the impedance.